Because the superconduction phenomenon reduces electric resistance to zero, it has become necessary to develop high-speed and low-power consuming superconducting transistors utilizing this phenomenon, and several types of structures have been proposed. Among these is a voltage-driven superconducting transistor that can be driven easily because of its large input impedance, and low input loss. The development of such a device is particularly desirable.
FIG. 3 is a cross section of a conventional voltage-driven superconducting transistor. The superconducting transistor has a structure in which a source 11 and a drain 12, which are superconducting electrodes, are disposed via an arsenic ion-driven part 15 on an Si single crystal substrate 14. Moreover, a gate 13, which is insulated by a gate oxide film between the source and the drain, is also disposed, the gate being covered by a side insulation film 17 and an overhang 16. In this case, for example, the source and drain are made of Nb, the gate 13 is made of polycrystalline Si, the overhang 16 and side insulation film 17 are made of Si.sub.3 N.sub.4, and the gate oxide film 18 is made of SiO.sub.2.
A region under the gate oxide film 18 is called a channel 19.
In the conventional superconducting transistor described above, the Cooper pairs, whose length is about the same as the coherence length, penetrate through the channel from the source and the drain, so that the coherence length is modulated by changing the strength of the impressed voltage on the gate so that the source and drain can be linked using these Cooper pairs.
Since this coherence length has conventionally been about several tens of nm, conventional superconducting transistors have required that the gate and drain be drawn closer together so that they are spaced apart only about 0.1 microns.
FIG. 4 is a line diagram showing the current-voltage characteristics of a conventional superconducting transistor. The horizontal axis represents the source-drain voltage while the vertical axis represents the source-drain current, and the diagram shows the current-voltage characteristics when the gate voltage varies from 0.5V to 1V and 2V.
Besides the voltage-driven superconducting transistor described above, current-driven superconducting transistors have also been proposed, although there have been no reports of such transistors with good characteristics, since application of current results in heat generation and only a small current gain.
As described above, conventional voltage-driven superconducting transistors require that the gate length (distance between the source and the drain) be 0.1 microns or less because the coherence length is short. Furthermore, in superconducting transistors that use oxides to make the superconductor, the gate length must be even shorter because the coherence length .lambda. in an oxide superconductor with a critical temperature Tc of 40 K or higher is less than several nm. Manufacturing a device with such a short gate length is extremely difficult. In addition, the distance between the source and the drain becomes so small that the withstand voltage between this source and the drain when an element is in an off state also becomes very small, making it very difficult to bond it with a conventional semiconductor element.